1. Field of the Invention
The present invention relates generally to a system for the carrying-in of a cassette for substrates to be processed. More specifically, the invention relates to a system for carrying, e.g., a wafer cassette, which holds therein a plurality of semiconductor wafers, in a processing unit, e.g., a load-lock chamber of a vacuum processing unit.
2. Description of the Prior Art
Semiconductor fabrication processes include vacuum processings, such as etching, thin film deposition, ashing and sputtering. In recent years, a single wafer vacuum processing unit called a cluster tool, which increases throughput by connecting a plurality of vacuum processing chambers and cassette chambers to a common transfer chamber, has been made fit for practical use.
FIG. 11 shows a conventional vacuum processing unit of this type. In FIG. 11, reference number 1 denotes a transfer chamber, and reference numbers 11 through 13 denote vacuum processing chambers. In addition, reference numbers 14 and 15 denote cassette chambers serving as load-lock chambers, and S denotes a cassette stage. In this system, an operator sets a wafer cassette (which will be hereinafter simply referred to as a "cassette") C, which houses therein, e.g., 25 wafers, on the cassette stage S so that the wafers are arranged horizontally (or substantially horizontally). Then, while the attitude of the cassette C is maintained, the cassette C is carried in the cassette chamber 14 or 15 by means of a transfer arm (not shown) provided between the cassette C and the cassette chamber 14 or 15. Thereafter, the wafers in the cassette chamber 14 or 15 are carried in the vacuum chamber 11, 12 or 13 by means of a transfer arm (not shown) provided in the transfer chamber 1, and a predetermined vacuum processing is carried out therein.
As shown in FIG. 12A, the cassette C is a container of a resin for holding therein a plurality of wafers W which are arranged vertically in parallel. The cassette C has an opening on the upper side for taking the wafers in and out. The inner wall of the cassette C is formed with a plurality of slots for receiving therein the edges of the wafers W. It is required to set the cassette C in the cassette chamber 14 or 15 so that the transfer arm in the transfer chamber 1 can access to the wafers W in the cassette C, i.e., so that the wafers W are arranged substantially horizontally as shown in FIG. 12B and the opening faces the transfer chamber 1. Therefore, in the cassette stage S outside the cassette chambers 14 and 15, the wafers W must be horizontally arranged and the opening must face doors 14a and 15a of the cassette chambers 14 and 15.
Since the operator carries the cassette while the wafers W are arranged horizontally, the operator must turn its wrist to the front by about 90 degrees while holding the cassette C. However, the cassette C housing therein 25 wafers is very heavy, e.g., about 4 kg when the size of the wafer is 8 inches, and greater than 10 kg when the size of the wafer is 12 inches. Therefore, this operation is an undesired human engineering operation since excessive load is applied to the operator's wrist.
As an example of technique for rotating the cassette C, U.S. Pat. No. 5,507,614 disclosed a cassette holder mechanism, which is schematically shown in FIG. 13. As schematically shown in FIG. 13, a L-shaped member 103 has two supporting surfaces 101 and 102, which are perpendicular to each other. A cassette C is mounted on one of the supporting surfaces 101 and 102 of the L-shaped member 103. The L-shaped member 103 is rotated by about 90 degrees by means of a motor 104 to rotate the cassette C.
If this cassette holder is arranged on the cassette stage S, it is not required to turn the operator's wrist. In this case, since it may be required to provide a cassette holder mechanism in addition to the transfer arm for carrying the cassette C in and out of the cassette chamber 14 or 15, an expensive transfer mechanism may be required. Therefore, it is considered that the cassette holder mechanism is combined with the doors 14a and 15a of the cassette chambers 14 and 15, i.e., the supporting surfaces of the cassette holder mechanism are also used as the doors, so that after the cassette C is mounted on the supporting surfaces, the L-shaped member is rotates so as to cause the cassette C to rise at the same time that the cassette C is carried in the cassette chamber 14 or 15.
However, with this construction, since the driving portion of the cassette holder mechanism is introduced into the cassette chamber, there are problems of contamination due to particle and contamination due to lubricating oil. In addition, an O-ring serving as a sealing material is provided between the door 14a or 15a and the edge portion of the carrying-in port of the cassette chamber 14 or 15, and the O-ring is deformed and the door 14a or 15a is drawn when the cassette chamber 14 or 15 is evacuated after the door 14a or 15a is closed. In view of this point, the mechanism is complicated. Moreover, in generally, clean air always flows from the top to prevent particle contamination even if the cassette C is supported on the supporting surface 101. However, since the supporting surface 101 also serves as a door and is a plate, it is difficult to flow the clean air as a laminar flow. Therefore, it is not desired that the cassette holder means is combined with the doors 14a and 15a of the cassette chambers 14 and 15.